1. Field of the Invention
The present invention relates to a CT (computerized tomography) imaging apparatus and related method for measuring local cerebral blood flow in which the local cerebral blood flow is detected by inhaling a tracer such as Xenon or Krypton into a patient under examination within a short time period and by employing an X-ray CT imaging apparatus.
2. Description of the Prior Art
A new non-invasive technique of cerebral blood flow measurement is recently known in the field, e.g., Radiology "Imaging of Xenon-enhanced cerebral blood flow with high resolution CT" issued in October, 1984 to Teeter and Colsher.
Conventionally, measurement of local cerebral blood flow is generally performed in the following manner.
A predetermined slice of the head of a patient under examination is scanned plurally over time during tracer inhalation so as to obtain tomographic images. A time-lapse variation of tracer concentration in cerebral regional tissue (to be referred to as Ci(t) hereinafter) and a time-lapse variation of tracer concentration in an artery (to be referred to as Ca(t) hereinafter) are obtained based on the obtained tomographic images.
Cerebral blood flow parameters, i.e., .lambda.i (partition coefficient), ki (build up rate) and fi (blood flow rate) are calculated based on the obtained variations Ci(t) and Ca(t) by the following equation (Kety-Schmidt equation) in accordance with the Fick principle: ##EQU1## or EQU (d/dt)Ci(t)=ki{.lambda.i.multidot.Ca(t)-Ci(t)} (1')
where Ci(T): tracer concentration of tissue i at time instance T
The above method uses a build up curve during tracer inhalation to obtain variations Ca(t) and Ci(t). For this reason, in order to obtain the correct values of the parameters .lambda.i, ki and fi, particularly, the partition coefficient .lambda.i of white matter, a tracer inhalation time of as long as 20 to 30 minutes is required. When the tracer inhalation time is long, the tracer cost is increased, a load on a patient under examination and an operator is increased, and the carbon dioxide gas content in blood of the patient under examination (PaCO.sub.2) is increased. In addition, the blood flow rate of the patient under examination is reduced from a normal level due to anesthetic effects of the tracer.
The variation Ca(t) is normally obtained by monitoring the tracer concentration in expiratory gas. When the variation Ca(t) is converted into a value according to the scale of the variation Ci(t), arterial blood must be sampled at least twice before tracer inhalation and twice after tracer saturation. This method cannot therefore be a non-invasive examination in a strict sense, and the operation procedures are complex.
Recently, cerebral blood flow measurement has become indispensable to diagnosis of neurotic diseases, selection of therapy, and assessment of therapeutic effect. However, as described above, the conventional measurement method is difficult to perform and provides only a low precision, resulting in an impractical method.
It is an object of the present invention to provide a CT imaging apparatus for measuring local cerebral blood flow which can obtain cerebral blood flow parameters with high precision within a short tracer inhalation time and without requiring blood sampling.